This invention relates to a device for detecting document image density for image forming apparatus, and more particularly, to a device which detects a quantity of light reflected from an original to be copied by, for example, an electronic copier, thereby detecting the image density of the original.
The quality of a copy image obtained by electronic copiers has been improved year by year. This owes to the improvement of material of a photosensitive drum or the improvement of material of toner. The improvement of a copy image also owes to mechanical improvements such as a correct adjustment of the distance between a photosensitive drum and a developing device. Further, the improved quality of a copy image is attributable to the use of an automatic exposure device which enables an electronic copier to diagnose the image density of an original. More specifically, the automatic exposure device detects the image density of an original and then controls an exposure light source, the voltage applied on the photosensitive drum or bias voltage of the developing device, in accordance with the image density detected, thereby achieving desired copying effects.
Hitherto, a photoelectric element has been used to detect the image density of an original document. The photoelectric element is so disposed below the document as to receive light reflected from the document surface. The photoelectric element can receive but light reflected from a portion of the document. It has been therefore inevitable that illumination light is controlled by the exposure control on the basis of the image density of only a portion of the document, not by the average image density of the entire document. If the photoelectric element is disposed below a central portion of the document in the width direction thereof, it receives light mostly from the central portion of the document and hardly receives light from edge portions of the document. In this case, the image density of the edge portions of the document is practically ignored in the exposure control. Therefore, proper exposure control, is difficult to accomplish. Depending upon the kind of document, particularly the size thereof, it may be impossible to obtain a copy of a desired image quality.